Anti-lock braking systems have now progressed to the point where they are standard on many vehicles. The use of traction control systems is now becoming increasingly widespread, and it is anticipated that their use would parallel that of anti-lock braking systems. In both systems, which may be termed "vehicle control systems," rapid deployment of brake calipers or brake shoes are necessary in order to perform the intended control function. In anti-lock braking systems, when locking of the wheels due to over-application of brake pressure or loss of traction due to the nature of the surface, i.e., gravel, ice, or snow, is encountered, the automotive braking system rapidly pulsates the brakes between an off and an on condition, allowing maximal retention of braking ability while yet retaining the ability to steer the vehicle in a stable fashion. In traction control systems, loss of traction in a driving wheel is countered by a momentary application of brake pressure, thus restoring traction. In either case, high pressure systems are desirable to affect the rapid changes necessary to achieve the desired control.
During anti-lock operation, it is necessary to rapidly decrease brake pressure by pumping brake fluid from the brake cylinders back to the master cylinder. This is necessary both for decreasing brake pressures and for having this dumped fluid available for subsequent antilock cycles in a stop. The motor driven high pressure pump is actuated only when the need for high pressure brake releases is sensed by the circuitry associated with anti-lock braking system or traction control system, as the case may be.
A typical anti-lock braking system is shown schematically in the above-referenced related patent application. In that system, hydraulic fluid from the brake pedal actuated master cylinder flows through a line through a normally open isolation solenoid valve to a brake caliper slave cylinder. Except for the presence of the additional normally open isolation valve, the system thus far described is similar to the normal braking system of the automobile. In an anti-lock brake system, detection of a lock condition actuates a high pressure pump and closes the solenoid actuated isolation valve. At the same time, a solenoid actuated hold/dump valve is opened, allowing pressure to bleed from the brake cylinder to the low pressure accumulator. The brakes are thus momentarily released. The low pressure accumulator allows quick initial dumping or decrease in brake pressure. The pump, however, empties the low pressure accumulator to allow continued decrease of brake pressure if needed and also pumps the brake fluid back to the master cylinder for subsequent needed antilock cycles of a stop. To reapply the brakes, pressure from either/or the master cylinder or the high pressure pump is diverted to the brake cylinder by opening the isolation valve and closing the hold/dump valve, once again increasing braking pressure. This cycle repeats itself rapidly as needed, resulting in rapid increases or decreases of brake pressure thus achieving maximal braking while avoiding a locked condition. Although this system is highly effective, it may be subject to noise and vibration due to the high pressure pulses emanating from the high pressure pump, as well as the pressure spikes and rebound pulses emanating from the isolation solenoid and the dump valves. In order to minimize these effects, it has proven useful to place an attenuator on the outlet side of the pump between the pump outlet and master cylinder. The combination of the compressible hydraulic fluid or elastomer within the attenuator cavity and a reduced diameter orifice adjacent thereto in the line leading therefrom, attenuates the pressure pulses and vibrations emanating from the pump.
Commonly used hydraulic systems in anti-lock and traction control systems utilize split or divided systems in which one portion of an opposed, dual piston pump supplies hydraulic fluid to two of the vehicle wheels, while the other piston of the high pressure pump supplies high pressure hydraulic fluid similarly to the other half of the braking system, i.e., the other two wheels. The braking circuit to each wheel is generally as described above, with the addition of integrated traction control requiring basically additional control valves and a high pressure accumulator associated with the hydraulic circuits in that portion of the system.
The various control valves, generally solenoid actuated valves, and the attenuators, low pressure accumulators, high pressure accumulators, if any, and high pressure pump elements are all commonly assembled in a single housing of extruded aluminum into which the various components are located in appropriately machined bores. Additional internal bores provide the requisite hydraulic circuit interconnections. Even though the body or housing of the hydraulic control unit is made of light alloy material, its size, and the size of the components located therein, many of whose parts are constructed of steel, still represent a significant amount of weight in a vehicle. Significant weight savings can be accomplished by reducing the overall size of the control unit, and additional savings can be effected by reducing the weight of the component parts of the various accumulators, attenuators, solenoids, and high pressure pump components.
In addition to the desirability of effecting weight savings by reduction of size and number of components, it is further desirable to reduce manufacturing costs by both reducing the number of components as well as aiding in ease of assembly of the components.